1. Field of the Invention
The present invention relates to a control apparatus and a control method for an automatic transmission. In particular, the invention relates to a technique for shifting gear by setting two frictional engagement elements to a disengaged state from an engaged state, and by setting other two frictional engagement elements to an engaged state from a disengaged state.
2. Description of the Background Art
Conventionally, automatic transmissions having a gear train composed of planetary gears are known. In such automatic transmissions, gears are implemented in accordance with combinations of frictional engagement elements that are set to an engaged state among a plurality of frictional engagement elements. As automatic transmissions are manufactured with multiple gears (as the number of gears being implemented increases), the shifting pattern varies as well. Shifting may be implemented gear by gear, or it may be implemented as downshifting from sixth gear to third gear, skipping one or more gears. When shifting is implemented gear by gear, it can often be achieved just by changing one of the plurality of frictional engagement elements being engaged. On the other hand, when shifting is implemented skipping one or more gears, it may not be achieved if all the engaged frictional engagement elements are changed.
Japanese Patent Laying-Open No. 2001-132835 discloses a shift control apparatus for automatic transmissions capable of preventing retardation of shifting while minimizing shock during shifting in which two frictional engagement elements are caused to enter a disengaged state from an engaged state, and other two frictional engagement elements are caused to enter the engaged state from the disengaged state. The shift control apparatus disclosed in Japanese Patent Laying-Open No. 2001-132835 controls shifting of an automatic transmission, in which actuation of four frictional engagement elements is required for shifting from a first gear to a second gear, the first gear is achieved by engagement of first and second frictional engagement elements, and the second gear is achieved by engagement of third and fourth frictional engagement elements. The shift control apparatus includes a shift control portion in which: disengagement of the second frictional engagement element is started after disengagement of the first frictional engagement element is started; engagement of the fourth frictional engagement element is completed after engagement of the third frictional engagement element is completed; and disengagement of the second frictional engagement element is started before engagement of the third frictional engagement element is completed. The shift control portion starts engagement of the third frictional engagement element, and thereafter starts disengagement of the second frictional engagement element, and then completes engagement of the third frictional engagement element. Disengagement of the first frictional engagement element and engagement of the third frictional engagement element are simultaneously started.
According to the shift control apparatus of the publication, disengagement of the second frictional engagement element is started after disengagement of the first frictional engagement element is started; and engagement of the fourth frictional engagement element is completed after engagement of the third frictional engagement element is completed. Thus, it becomes possible to shorten the period of a shift state in which all of the four frictional engagement elements is slipping, and increase the period of a state in which one frictional engagement element is engaged. Therefore, the shifting state of the transmission mechanism is prevented from progressing disorderly due to slipping of the four frictional engagement elements. As a result, occurrence of great shock when shifting is completed can be suppressed. Further, disengagement of the second frictional engagement element is started before engagement of the third frictional engagement element is completed. Thus, shifting can be implemented without incurring a state where two frictional engagement elements are completely engaged simultaneously. This can prevent temporal stopping of the progress of shifting that invites shifting in two stages. As a result, retardation in shifting can be prevented. Still further, disengagement of the second frictional engagement element is started after engagement of the third frictional engagement element is started and before the engagement is completed (complete engagement). Thus, by the slip state just after starting disengagement of the second frictional engagement element and the slip state just before completing engagement of the third frictional engagement element, the engine can be revved. Therefore, retardation in shifting associated with the start of engagement of the third frictional engagement element can be offset.
However, in the shift control apparatus of Japanese Patent Laying-Open No. 2001-132835, since the engine is revved when the second frictional engagement element and the third frictional engagement element are in the slip state, the increase in the engine speed (the turbine speed of the torque converter) is suppressed. Therefore, the increase in the engine speed is slow. Accordingly, it may not be possible to completely offset the retardation in shifting associated with the start of engagement of the third frictional engagement element. In this case, the time required for shifting may be increased.